场景:当你在某个界面请求网络数据的时候,用户不愿意等待点击了返回按钮,此时在Block当中用如下的方式使用weakSelf的话,有可能会奔溃(因为在并发编程的情况下,虽然在if判断的时候weakself不为空,但是不保证if语句里面的weakself不为空),所以为了安全起见要加上strongSelf(参考)。
if (weakSelf != nil) {
// last remaining strong reference released by another thread.
// weakSelf is now set to nil.
[myArray addObject:weakSelf];
}
那为什么加上strongSelf就可以保证self不为空了?
因为如果self有效,那么strongself就是利用了循环引用的特性保证了在block执行的时候self对象不会被析构,strongSelf是局部变量执行完成后就会释放
接下来用个Demo验证下,在FirstVC的ViewDidLoad里面添加以下代码
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (0.5* NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[self.navigationController pushViewController:[SecondVC new] animated:NO];
}); //0.025 - 0.03
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (0.55 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[self.navigationController popViewControllerAnimated:NO];
});
然后在 SecondVC的 里面添加以下代码
typedef void(^TTestBlock)(int result); @interface SecondVC ()
@property (strong, nonatomic) TTestBlock aBlock ;
@end @implementation SecondVC - (void)viewDidLoad {
[super viewDidLoad]; [self testForBlock];
} - (void)testForBlock{ __weak typeof(self) wSelf = self;
self.aBlock = ^(int result) { NSLog(@"testForBlock:%@",wSelf);
dispatch_async(dispatch_get_global_queue(0, 0), ^{ //没有下面这一行的话,testForBlock2 中的self为空
__strong typeof(wSelf) strongSelf = wSelf; NSLog(@"testForBlock1:%@",wSelf); [NSThread sleepForTimeInterval:1]; //__strong typeof(wSelf) strongSelf = wSelf; //放在这儿的话也wSelf会为空
NSLog(@"testForBlock2:%@",wSelf);
});
} ;
self.aBlock(0);
}
@end
注意: